This invention relates to dynamometers, and is more particularly directed to a system for automatically measuring and recording engine torque and horsepower over a range of engine speed values, so that the performance characteristics of an engine can be determined.
Existing dynamometer instrumentation systems operate according to a basic principle. An engine is brought to a predetermined engine speed and stabilized at a given RPM while holding a torsional load on the engine's rotary output shaft. The engine torque value and the RPM value are recorded, either manually (with pad and paper) or electronically. Then the engine is brought to another engine speed and stabilized, and the torque and engine speed values are recorded. This is a rather slow process, so usually there are no more than about 5 or 6 data points taken. This usually means recording only a single torque value for every 500 RPM over a very narrow speed band.
Computer controlled instrumentation has been employed in connection with this general method. Unfortunately, the method still involves taking a very limited number of data samples, and then over widely separated RPM values. This yields simple point value readings, as before. This method is subject to wide variances from one test run to another, and this is largely due to statistical fluctuations of the measured data. The previous method more or less assumes an engine that delivers smooth, steady power during every aspect of a power cycle. In a real internal combustion engine, however, the power is produced in pulses (during the power strokes) with each cylinder contributing zero or negative values of torque being applied during intake, exhaust and compression strokes. Each engine stroke will be inconsistent from cycle to cycle, producing natural fluctuations in the torque impulses. As should be understood from this, the torque, power, and RPM delivered from the engine shaft vary even during steady state conditions. Consequently there is a rather large uncertainty factor in the prior art method readings. This uncertainty can be on the order of about 1% reading.
Attempts to measure torque (and power) during engine acceleration have not presented reliable and consistent results. The reasons for this have not been appreciated, even though in hindsight it might seem obvious. The engine and the dynamometer have rotational inertia, and this absorbs some of the engine torque when acceleration takes place. The rotational inertia releases power and torque when the engine is decelerated. Consequently, torque and power readings are below the true values during acceleration, but above them during deceleration. This variation is a simple first order relationship, the torque loss owing to acceleration being directly proportional to the amount of acceleration. There is one other factor limiting the reliability and accuracy of the results during an acceleration test run. Conventional instrumentation techniques obtain data by taking quick "samples" of the signal and rely on having this signal being filtered to smooth out the torque pulses and other fluctuations. However, this filtering also causes the signal to lag behind during changes and engine accelerations thus creating a false and misleading result.